1. Field of the Invention
The present invention relates to a supervisory signal transmitter and method for use with a WDM communications system.
2. Description of the Related Art
Nowadays, as information processing technologies have developed and advanced, the demand has increased for constructing a multimedia network which can handle various types of data in the field of communications as well. Such a multimedia network needs to be able to transfer large amounts of image data simultaneously with sound data. To this end, the network's signal transmission rate must be increased.
Optical communications networks using optical fibers are in the spotlight as such a network and have been researched and developed intensively. In particular, a light wavelength-division multiplexing (WDM) communications system is considered to be promising as a method of accommodating multiple channels with one optical fiber. To make such a WDM communications system feasible, it is necessary to use a technique for network supervision and control. As such a technique, a method will be used which supervises and controls repeaters and the like which make up a network by causing a supervisory signal (SV signal) used for network supervision and control only to flow through the network.
In a one-wave transmission system, the SV signal is generally transmitted superimposed upon a main signal. In the WDM communications system, it is promising to transmit the SV signal in a dedicated wavelength (channel). An arrangement for transmitting an SV signal to all repeaters that make up a network is proposed in, for example, Japanese Patent Application No. 9-065231.
FIG. 1 is a schematic diagram of the proposed SV-signal transmission system.
Stations A to D each transmit a light signal modulated with data to be sent. In the WDM communications system, different wavelengths are assigned to different channels and light signals of different wavelengths are wavelength-division multiplexed for transmission over a transmission path. The transmission path is equipped with a number of repeaters that amplify light signals attenuated as a result of transmission over the path, thereby allowing long-distance transmission.
In the middle of the transmission path, there are provided branching units 60 and 61 each of which separates a light signal of a specific wavelength from the wavelength multiplexed light signals and sends it over a separate transmission path. For example, a light main signal sent from the station A is separated by the branching unit 60 into a light main signal to be directed to the branching unit 61 and a light main signal to be directed to the station B.
The light main signal directed from the branching unit 60 to the station B is terminated by the station B. On the other hand, the light main signal directed to the branching unit 61 is further separated by the branching unit 61 into a light main signal to be directed to the station C and a light main signal to be directed to the station D. Each of the stations C and D terminates the received light main signal.
In the currently proposed SV-signal transmission system, for example, the station A is set to send and terminate an SV signal, and the SV signal and a light main signal are multiplexed together for transmission. At the branching unit 60, the SV signal is separated and then directed to the station B as shown by an arrow in the figure. At the station B, the SV signal is looped back to the branching unit 60. At the branching unit 60, the SV signal and the light main signal are multiplexed together for transmission to the branching unit 61. At the branching unit 61 as well, the SV signal is separated and then sent toward the station C. At the station C as well, the SV signal is looped back to the branching unit 61. At the branching unit 61, the SV signal and the light main signal are multiplexed together and directed to the station D. At the station D, the SV signal is likewise looped back to the branching unit 61. After that, as in the case of transmission from the station A to the station D, the SV signal is separated and multiplexed at the branching units 60 and 61, looped back to the corresponding branching unit at the stations B and C, and finally terminated by the station A.
By looping back the SV signal at the stations B, C and D in this manner, an SV signal-only path can be formed which begins at the station A and ends at the station A. By sending the SV signal over this path, all the repeaters on the network can receive the SV signal as is evident from FIG. 1. Thus, the provision of only one channel dedicated to the SV signal allows supervision and control over all the repeaters on the network. If it is impossible from the viewpoint of network configuration to form one path so that it can pass through all the repeaters, SV signals each assigned a different wavelength may be transmitted.
To acquire repeater's states (its output, the temperature of a light source used, etc.) from a specific repeater, data containing an identification number that identifies the specific repeater is transmitted using an SV signal as a carrier. That is, the data is represented as a digital signal by turning the SV signal on and off. However, since the SV signal is transmitted combined with a light main signal, the power of all the light signals containing the main signal and the SV signal will vary each time the SV signal is turned on or off. The transmission characteristics of an optical fiber depend on the power of a light signal which propagates therethrough. As a result, the transmission characteristics will vary each time the SV signal is turned on or off, degrading the performance of the WDM communications system.
One way to make the power of all light signal that propagates through an optical fiber constant will be to transmit a direct-current light in place of the SV signal when it is off. By regulating the output of the direct-current light properly, it becomes possible to keep the power of all the light signals constant so that the transmission characteristics of the WDM communications system will be placed in the stable state.
However, this method is not very desirable from the viewpoint of circuit arrangement.
Problems with the direct-current light-based system will be explained with reference to FIGS. 2A and 2B.
FIG. 2A shows the manner in which the direct-current light is output in the intervals when an SV signal is off.
In FIG. 2A, it is assumed that the SV signal is turned on and off at a frequency of 10 MHz and a command signal whose 1s and 0s are represented by the presence (on) and absence (off) of the SV signal has a frequency of 10 to 20 kHz. In order to transmit direct-current light having the same power as that of the SV signal propagating through optical fiber, it is required to set the power of the direct-current light lower than the maximum power of the SV signal (indicated as ON on the ordinate in FIG. 2A).
If, for example, the SV signal is a rectangular pulse train having a duty ratio of 50%, then it will be required to set the power of the direct-current light to one-half of the maximum power of the SV signal as shown in FIG. 2A. By so doing, a light signal which is constant in power on average can be transmitted all the time over optical fiber, providing stable transmission characteristics.
The problems with the transmission of direct-current light in the absence of the SV signal will be described with reference to FIG. 2B.
In an actual transmitter arranged to transmit direct-current light in the absence of an SV signal as shown in FIG. 2A, it is desirable that switching from the SV signal to the direct-current light be made instantaneously. In an actual circuit, however, since a laser light source used has delayed response, switching from the SV signal to the direct-current light cannot be made instantaneously, with the result that the power will attenuate gradually with time as shown in FIG. 2B. In FIG. 2B, this phenomenon is indicated as a slope due to the delayed response of the circuit.
Such a phenomenon appears as noise in the optical fiber, which results in the degradation of the transmission characteristics of the optical fiber and also causes errors in received data in the repeaters which receive the SV signal. In addition, problems arise in the circuit arrangement for generating the direct-current light as well. That is, describing in terms of the example of FIG. 2A, a circuit would be required which does not respond to the SV signal which is turned on and off at a repetition frequency of 10 MHz but responds to the command signal which is represented by the presence and absence of the SV signal and corresponds in frequency to 10 to 20 kHz. To construct such a circuit, a narrow band filter is required. Thus, the direct-current light generating circuit is complex in arrangement, large in size, and costly.